1. Field of the Invention
This invention relates generally to the conversion of biomass into useful intermediates and/or end products. More particularly, the invention relates to processes and apparatuses for converting solid particulate biomass into gaseous and liquid products that may be utilized for transportation fuels, petrochemicals, and/or specialty chemicals.
2. Description of the Related Art
With its low cost and wide availability, biomass is an ideal feedstock to produce “green” liquid fuels. By utilizing biomass as a feedstock, our dependence on fossil fuels for energy needs may be reduced.
In an attempt to harness the energy potential of biomass, many processes have been developed that convert biomass into fuels and/or specialty chemicals. Examples of conventional biomass conversion processes include combustion, gasification, slow pyrolysis, liquefaction, and enzymatic conversion. Despite their potential to convert biomass, many of the aforementioned processes have exhibited drawbacks. For example, combustion is restricted to immediate thermal applications and does not produce any useful products. Gasification requires a large amount of energy to provide the necessary high temperatures (800-1,300° C.) needed to produce syngas, a low energy fuel gas with limited uses. Both slow pyrolysis and liquefaction require long residence times in their reactors and produce low quality yields of the desired liquid or gaseous products. Furthermore, the liquid products of slow pyrolysis and liquefaction often require considerable secondary upgrading before being used as a biofuel. Finally, enzymatic conversion takes considerable time, requires expensive equipment, and its products require extensive upgrading in order to be commercially viable.
Fast pyrolysis is generally recognized as offering the most promising route to the conversion of solid biomass materials into biofuels and/or specialty chemicals. Fast pyrolysis relies on a fast heating rate of the biomass feedstock, a short residence time in the reactor, and rapid cooling from the reaction zone. Consequently, a larger yield of the desirable liquid product is produced, while the yield of undesirable reaction products, such as char, coke, and ash, is reduced. The liquid product produced by pyrolysis is commonly referred to as “pyrolysis oil;” however, when derived from biomass it is generally called “bio-oil.” Bio-oil has many useful end-use applications, such as in transportation fuels, petrochemicals, specialty chemicals and/or as food additives (liquid smoke).
Despite its recognition as an advantageous process to produce bio-oil from biomass, much research is still being undertaken to determine which reactor configuration may best facilitate the fast pyrolysis reaction. Lately, the focus has been on ablative reactors, cyclone reactors, and fluidized reactors to provide the fast heating rates needed for fast pyrolysis. Fluidized bed reactors, including riser reactors and fluidized stationary bed reactors, have been the basis of much research due to their ubiquitous availability in existing infrastructure based on their use in petroleum refineries.
Previous fast pyrolysis processes utilizing a riser reactor have produced lower quality bio-oils. For example, U.S. Pat. No. 5,961,786 discloses a process for converting wood particles to a liquid smoke flavoring product (bio-oil). The process uses a transport reactor, with the heat being supplied by hot heat transfer particles, such as sand, sand/catalyst mixtures, or silica-alumina catalysts. Although this process produces a relatively high bio-oil yield in the range of 50 to 65%, the produced bio-oil had properties that were desirable for “liquid smoke” food flavorings (low pH, high oxygen content, and browning propensity), but that are undesirable for liquid fuels.
In light of the foregoing, there exists a need for an efficient and economical biomass conversion process that provides high yields of high quality liquid fuels, such as bio-oil.